Headroom is margin: how many dB remain between where the signal sits and where the system runs out. In digital audio the ceiling is 0 dBFS and it is absolute; anything that tries to pass it clips. Analog gear has softer ceilings that distort progressively, which is why “running hot” was once a sound and is now mostly a mistake.
Mix engineers leave headroom deliberately, keeping peaks a few dB under the ceiling so mastering has room to work and intersample peaks do not sneak past the meters. Live engineers leave more, because a crowd and adrenaline always find the spare dB.
The concept generalizes to any reactive system. A visual pipeline has headroom too: if a normal kick already pushes every parameter to maximum, the drop has nowhere to go. Reserving range for the biggest moments is the same discipline as reserving dB for the loudest peaks, and systems designed without it feel flat exactly when they should feel huge.